Introduction to Data Communications
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21. Line Encoding

The waveform pattern of voltage or current used to represent the 1s and 0s of a digital signal on a transmission link is called line encoding. The common types of line encoding are Polar, Unipolar, Bipolar and Manchester encoding.

21a. Unipolar Encoding

Unipolar encoding has 2 voltage states with one of the states being 0 volts. Since Unipolar line encoding has one of its states being 0 Volts, it is also called Return to Zero (RTZ). A common example of Unipolar line encoding is the TTL logic levels used in computers and digital logic.

Unipolar line encoding works well for inside machines where the signal path is short but is unsuitable for long distances due to the presence of stray capacitance in the transmission medium. On long transmission paths, the constant level shift from 0 volts to 5 volts causes the stray capacitance to charge up (remember the capacitor charging formula 1-e-t/RC !). There will be a "stray" capacitor effect between any two conductors that are in close proximity to each other. Parallel running cables or wires are very suspectible to stray capacitance.

If there is sufficient capacitance on the line and a sufficient stream of 1s, a DC voltage component will be added to the data stream. Instead of returning to 0 volts, it would only return to 2 or 3 volts! The receiving station may not recognize a digital low at voltage of 2 volts!

Unipolar line encoding can have synchronization problems between the transmitter and receiver's clock oscillator. The receiver's clock oscillator locks on to the transmitted signal's level shifts (logic changes from 0 to 1). If there is a long series of logical 1s or 0s in a row. There is no level shift for the receive oscillator to lock to. The receive oscillator's frequency may drift and become unsynchronized. It could lose track of where the receiver is supposed to sample the transmitted data!

Receive oscillator may drift during the period of all 1s

Introduction to Data Communications
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